Manufacture of paper pulp



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. ez m wm, WVMWW 3,002,877 MANUFACTURE FF PAPER PULP Dan McDonald, Oswego, 111., assignor to Leona S. Jones, St. Louis, Mo. Filed Feb. 27, 1959, Ser. No. 796,172

(Ilaims. (Cl. 162-74) This invention relates to the manufacture from wood of cellulose fiber pulps including paper pulp. The pulp produced in accordance with this invention is suitable for use not only in the manufacture of various kinds of paper, but it may be used advantageously in the manufacture of other products containing or derivable from cellulose.

Heretofore, fibers obtained from wood for use in the manufacture of paper and other products have been produced primarily by a few well-known commercial processes, namely, the ground pulp process, also termed the mechanical pulp process, the sulfite process, the sulfate process, and the soda process. The latter three processes are classed as chemical pulp processes, in view of the fact that they include a digesting step for dissolving the lignins which bind the cellulose fibers.

In each of the above mentioned chemical pulp processes the wood is prepared for the process usually by very careful removal of the bark, followed by chipping of the wood into chips ranging generally from about to 1" in length. The chips are introduced into a digester along with the cooking liquor, the digester sealed, and steam is quite generally, though not always, introduced directly into the digester to raise the temperature of the liquor and to subject the digesting action to pressure. The cooking of the chips is allowed to proceed for a time interval that may range from two to six hours or more, the length of the period being dependent somewhat upon such factors as the type of Wood being processed, the quality of paper pulp desired, and other factors not necessary to mention here. Where steam is introduced directly into the digester, the condensate therefrom gradually dilutes the cooking liquor and this dilution must be taken into account in pre determining the strength of the cooking liquor at the start of the operation. In some instances, dilution of the cooking liquor by condensed steam is avoided by the use of steam coils or steam jackets for heating the digesters. During the digesting operation the lignins are gradually dissolved by the liquor and some of the terpenes are vaporized off from the wood.

An added cause of the dilution of the digester liquors in prior processes resides in the fact that the wood chips themselves introduce into the process a large volume of water due to their initial moisture content, which will perhaps average around 100% and may vary from 60% to far above 100% moisture content, figured on the dry basis. 1

After the cooking operation in the prior processes is completed the pulp is discharged from the digester and is washed in dilute liquors and with water to free the cellulose fibers, which eventually are segregated by screening and bleached if and as desired, involving sometimes as many as 6 to 12 successive bleaching operations. Bleaching whitens the fibers, but impairs their strength and diminishes the total pulp production.

Since the resins, gums and terpenes, commonly present in most commercial pulp woods, have not previously been removed from the Wood chips prior to the beginning of the digesting action, their presence retards the penetration of the chips by the liquor and impedes its digesting action in the lignins. The resins, when present, contribute to the darkening of the digesting liquor and cellulose fibers, thus increasing the difficulty and ex- "ice pense of liquor recovery, and necessitating drastic bleaching of the darkened fibers.

Whereas, in the prior conventional chemical pulp processes, the wood chips were subjected to digestion while still containing their normal content of moisture, resins, gums and terpenes, in accordance with my invention the wood chips are reduced in moisture to a controlled degree and are freed of resins, gums and terpenes before digestion begins, and the chips are, furthermore, thoroughly permeated and saturated by this liquor at the outset of the digestion action. The moisture is removed by evaporation to the extent desired, the resins, gums, and terpenes are removed by solvent extraction, and the digestion may be performed with any of the known digester liquors or any liquor which is suitable for digestion of the lignins and which preferably does not have too destructive effect on the cellulose. The invention is adaptable either to batch or continuous operation, although the latter is preferred.

For the sake of conciseness, wherever the specification or claims mention resins without specific reference also to gums and terpenes, the term resins is intended to include the gums and terpenes. Also, reference in the specification and claims to cooking or digester liquor means any liquor suitable for digesting the lignins.

Where the term moisture free is used it is intended that moisture be removed sufiiciently to make possible extensive resin extraction and penetration of solvent into the chips to a degree where, when the solvent is evaporated from chips in the presence of cooking liquor, the liquor will be drawn into the voids in the chips caused by solvent removal.

A preferred method of practicing my invention is briefly described as follows. Moisture is first extracted from the chips while submerged in a resin solvent selected from the solvents named herein, whereby the space in the chips originally occupied by moisture becomes saturated by the solvent. Thereupon the resins, if any, present in the chips become substantially completely dissolved in the solvent and move out of the chips. The solvent carrying the resins is drained away from the chips, leaving the chips saturated with solvent only. The chips are then submerged in a heated aqueous solution of digester liquor at a temperature below conventional digesting temperatures. As the solvent then vaporizes from the chips, the digester liquor replaces the solvent and itself saturates the chips before any or substantial digestion begins. This is important. Thereafter, higher heat and pressure cause uniform and complete digesting reaction with all fibers and lignins at the same time. In this manner there should be little or no cellulose damaged or liberated in advance of the complete reaction, whereas in the prior conventional practice the outside of the chip is subjected to the destructive effect of the strongest liquor, which destroys, weakens and otherwise damages such cellulose.

It should be noted that some prior experimenters, who solvent-extracted resin from the chips, thereafter stripped their solvents from their chips with steam and thereby reloaded their chips with water and air prior to contact with the digesting liquor, and when digestion began only the outside fibers could be and were attacked initially by the liquor.

The process steps of my invention may readily be ccordinated consecutively in a continuous process in a manner which lends itself to economical operation in app-aratus which may be manufactured at a reasonable cost. Furthermore, the invention yields a number of advantages which perhaps are not at first apparent.

For example, cellulose fibers, after being solvent treated and then digested in accordance with this process will have in general a lighter color at the end of the digesting period than fibers produced heretofore by standard commercial processes.

Additionally, the cooking liquor emerging with the chips will contain a higher percentage of'dissolved solids due to the use of a smaller volume of liquor required in this puroeess per thousand pounds of chips weighed on the bone dry basis. Also, this percentage of dissolved solids should be constant on a continuous run of wood of the same type, regardless of the varying initial moisture content of such wood.

Furthermore, in accordance with this invention, not only can a uniform concentration of liquor be used but its initial concentration need not be as great as in conventional chemical processes, and since a less severe initial concentration can be used and the treatment by the digesting liquor be less prolonged the fibers themselves will be to a less extent injured or destroyed during the digesting stage of the process.

Another advantage residing in this process is that there can be less liquor used and less to recover.

The general object, therefore, of this invention is to provide a new process for the treatment of wood to produce therefrom cellulose fibers for paper making or for other uses to which wood fibers have been or may be devoted.

Various other objects and novel advantages of this invention have been or will be specifically mentioned hereinafter, or will become apparent from a perusal of the specification, in which a preferred manner of practicing the invention is described.

The drawings show, largely in diagrammatic form, appanatus which may be employed in the practice of the process described herein. It should be observed that since the drawings are in diagrammatic and condensed form, the drawings do not permit use of a uniform scale to show the apapratus and parts thereof in the relative sizes or preferred positions which they may have in commercial practice.

In the drawings,

I FIGURE 1 is a diagrammatic illustration of an immersion chamber and an associated condenser and still employed in the invention, and

FIGURE 2 is a diagrammatic illustration of a digesting chamber and associated chip feeding and discharge apparatus.

FIGURE 3 is a sectional view on line 33 of FIG. 1.

FIGURE 4 is a sectional view on line 4-4 of FIG. 3.

For the sake of clarity these sectional views show structure rather than mere diagrammatic lines.

FIGURE 5 is a diagrammatic illustration of a modified flow system for solvent and azeotrope vapors.

FIGURE 6 is a diagrarmnatic illustration of a modified form of apparatus which may be employed.

FIGURE 7 is a diagrammatic illustration of a modified form of apparatus which may be employed.

FIGURE 8 is a sectional view on line 83 of FIG. 6.

Referring further to the drawings, in FIG. 1 there is shown an elongated horizontally extending chamber generally designated as 11 in which is disposed an endless perforated belt or link type screen conveyor 12 which may conveniently be made, if desired, of a wire screen suitably reinforced in a manner not shown, and carrying thereon at spaced intervals transversely extending perforated paddles such as 13 rigidly secured perpendicularly to the surface of the belt.

A worm gear drive and motor 14- is indicated diagrammatically as the motive power for driving the belt 12 which is guided on a plurality of rollers 15.

It will be noted that in the lower portion of the chamber 11 there are four ridges or dams indicated as 16, 17, 18 and 19 extending transversely across the bottom of the chamber and thus segregating four separate liquid baths designated as A, B, C, and D.

Before undertaking a description of the function of the solvent baths A, B, and C, it may here be observed that the solvent employed may be selceted from among several having the requisite physical characteristic, as will be more fully discussed hereinafter. However, for the sake of brevity and as an illustrative example, the vari ous forms of the process herein disclosed will be described as employing perchlorethylene, this being one of the preferred solvents presently available in commercial quantities.

Perchlorethylene has a boiling point of 250 F., it is immiscible with water, it is a good solvent for the natural wood resins, it forms an azeotrope with water, having an azeotrope boiling point of about 190 F., and the ratio of solvent to water in this azeotrope vapor is about 5.3 pounds of the solvent to 1 pound of water. The latent heat of vaporization of perchlorethylene is about 90 Btu. and its specific gravity is about 1.6. It may be noted that the latent heat of vaporization of water is about 970 B.t.u. and water has a specific gravity of 1. Hence it requires about 1447 B.t.u. to evaporate 1 pound of water in this azeotrope vapor, that is, 970' plus 5.3 times 9G=1447.

Therefore, when describing the present invention, reference will be made to perchlorethylene as the solvent being used and the stated boiling points for solvent or azeotrope are those peculiar to perchorethylene. However, it should be recognized that if another solvent be substituted, the process steps must be regulated with due regard for the different physical characteristics of the particular solvent selected, as will be appreciated more readily after a perusal of this entire specification.

The solvent may be introduced initially through pipe 20, thence through valve 21, which may be opened as needed for the initial supply of solvent to the process and for purposes of replenishing the solvent, as may be necessary, the solvent then entering through pipe 22 and through the top of the chamber 11 whereupon it will then form and maintain the bath indicated as C. In plant procedure an automatic liquid level control would be used in regulating the addition of fresh solvent. A heating jacket 23 will be supplied with steam or still vapors, as later explained, and regulated to heat this solvent bath C well above the azeotrope boiling point but to less than 250 E, which is the boiling point at atmospheric pressure of the solvent.

Continuous supplying of the solvent, as later explained,

- maintains the liquid level in bath C at the level established by overflow pipe 24, through which the excess flows, thence through a water cooled tubular heat exchanger 25 and thence through the pipe 26 to form the bath B. The cooler 25 may employ water or any other 1 suitable coolant and may be regulated to assist in reducing the temperature of the hot solvent now flowing from bath C into bath B to a temperature range of F. to F. As a further cooling aid and to make sure that the bath B is maintained within the indicated temperature range, a cooling jacket 27 may be attached to the bottom of the chamber 11, and water or other coolant is flowed therethrough under a temperature control system, not shown, to aid in the control of the temperature of the bath B. Bath B is maintained at a level somewhat below the top of the darn 16 as a result of the continued supply of solvent from bath C, the level being regulated by overflow pipe 28, the solvent then flowing through the tubular heater 29 and through the pipe 30 into the bath A. Heater 29 may be regulatably heated either by steam or solvent vapors from the still, later described.

The bottom of the chamber 11 underneath the bath A is provided with a steam heated jacket 31, which, together with the heater 29, both of which Will have suitable thermostatic controls, not shown, serve to maintain the bath A well above the azeotrope boiling point but to less than 250 F. Additional means for maintaining the bath temperature may be provided if needed.

The wood chips are introduced into and move through the process steps in the following manner:

The wood chips will be brought to the hopper 32 continuously by some conveyor system, not shown, and which does not form a part of the invention. A barrel valve 3-3, continuously rotating, will feed the chips at a regulated rate downwardly into the chamber 11, while preventing in conjunction with chips in the hopper and barrel valve the unrestricted entrance of excessive amounts of air. The condensing action of the azeotrope vapor condenser and the action of a suction fan, both later described, may be utilized to maintain a slight negative pressure in the chamber 11, which is desirable in order to prevent any undue escape of the solvent into the surrounding atmosphere.

As the chips pass through the barrel valve 33 they may be directed by means of stationary or vibrating bars such as 34 or other suitable mechanism to fall upon the endless belt 12 in a layer of uniform depth transversely of the belt and be carried by the paddles 13- downwardly past the yieldable spring loaded feed guide 34 backed by a plurality of springs such as 35, thence into the bath. The screen wire belt is submerged in all of the baths as it moves therethrough, except when riding over the dams, and will therefore hold the chips submerged in each of the successive baths. The buoyant chips will be propelled through each bath between the successive paddles. Chips entering bath A will quickly be heated by the body of hot liquid solvent therein, causing the water to diffuse to the surface of the chips, whereupon this water will combine with the solvent or solvent vapor to form an azeotrope vapor whose boiling point will be at about 190 to 191 F., measured at or about atmospheric pressure. As the chips move slowly through the bath A the moisture therein will be driven out, down to about 7 to percent moisture content, if desired. There will be some penetration of the chips by the solvent as the moisture is removed. Moisture removal can be controlled and only sufiicient moisture need be removed to make possible in the later stages good resin extraction and uniform and substantially complete absorption of the cooking liquors.

As an alternative mode of feeding the chips into the tank 11, the hopper 32 could be positioned above bath B or bath C to drop the chips upon the top of the upper traverse of the belt 12, which could give the chips an opportunity to be pre-heated in the heated vaporous atmosphere before being immersed in bath A. Thus some water could be evaporated from them while so supported on the belt forming an azeotrope vapor with the solvent present in such atmosphere.

The azeotrope vapor willbe continuously exhausted through the conduit 36 and upon passing through the temperature regulated water cooled condenser 37 will be cooled to a temperature lower than 190 F. and therefore largely, if not wholly, condensed. This condensate of Water and solvent, accompanied by any noncondensible gases and uncondensed vapors, will then flow through pipe 38 into the closed gravity separator 3-9, where by the action of gravity the water will separate readily from the solvent because of the substantially higher specific gravity of the solvent. The Water will then flow through the overflow pipe 40 to any suitable point of discharge, or may be used in any desired manner, for example in the making up of cooking liquor or as a coolant in cooler 25.

The separated solvent will then flow through the overflow pipe 41 through a heater 42 where it may be indirectly heated by steam or otherwise to nearly 250 F. if desired, after which it will re-enter the chamber 11 as a liquid through the pipe 22.

Uncondensed vapors and non-condensible gases freed in the separator 39 may be drawn upwardly through pipe 43, the check valve 44 and pipe 45 into a carbon adsorber 46 by the action of the continuously running exhaust fan 47, which is connected by pipe 48 through the two-way valve 49 to the top of the carbon adsorber. The carbon granules in the adsorber will adsorb the solvent vapor, while the non-condensible gases will be discharged to atmosphere.

Periodically, as may be determined by calculation or from the exhaust of blower 47, or otherwise, when the carbon pack is substantially saturated with solvent vapor, valve 49 will be reversed to cut oif the suction from the blower 47, as exerted on the pipe &3, and steam under pressure will be introduced through the pipe 50 downwardly through the pipe 48 and through the carbon adsorber to strip the solvent as an azeotrope from the adsorber. The stripped solvent will be driven by the steam downwardly through the pipe 45, through the check valve 51, thence through the condenser 37 and into the gravity separator, whereupon it will then pass through the overflow pipe ti and be returned to the chamber 11.

When the carbon adsorber is sufficiently purged the valve 49 will be reversed to cut off the steam flow and the exhaust blower 47 will resume its operation of drawing the solvent vapor and non-condensible gases upwardly into the carbon adsorber. it may be stated that the carbon adsorber, when not saturated, will adsorb all of the solvent vapor entering so that the exhaust of blower 47 will not contain any of the solvent.

The chips will move through the bath A continuously at a regulated rate so that the function of driving the moisture out of them will be 80 to 90% complete or to the extent desired, after which the chips are then lifted over the dam in by the paddles on the endless belt and are then dropped into the bath B.

The bath B is maintained at a temperature below 190 F., preferably in the range indicated, that is l80l85 F. Such water vapor, together with any solvent vapor and azeotrope vapor that may be present and still remains in the chips will immediately condense and enable the solvent in bath B to move in and penetrate the chips thoroughly. The solvent at this temperature range, and any solvent which may previously have penetrated the chips in bath A, will begin the function of dissolving the resins which are contained in the wood chips.

The chips are continuously moved through the upper portion of bath B and then carried by the perforated paddles over the dam 17 without dragging any excessive amount of solvent with them and are then introduced into the bath C which is maintained at a higher temperature, between 190 F. and 250 F. In the hot bath C, to which resin-free solvent is continuously supplied by pipe 22, the chips will give up further moisture which forms an azeotrope vapor with the solvent, and the action of removing the resin from the chips will proceed more rapidly. As long as there is a higher concentration of resins in solution in the solvent inside of the chips than there is in the solvent outside of the chips in the bath C the difference in osmotic pressure will cause the resins to diffuse out from the chips into the bath. The chips remain long enough in the solvent bath C as they progress countercurrently therethrough so that the solvent substantially completely dissolves and removes from the chips the resins which they had previously contained. The continuous flow of solvent through bath C will maintain the resin content of the bath at such a low percentage that the osmotic pressure diiferential mentioned above may readily be maintained.

The endless belt will then continuously move chips rom the bath C over the dam 18 and into the entering end of the bath D. This latter bath is composed of an aqueous solution of a cooking liquor employed in the process. In selecting a cooking liquor for the process it will be borne in mind that if the chips being processed originally contained much resin they will at this stage be substantially resin free. I may choose to use a cooking liquor of the type heretofore commonly employed in the so-called sulfate process, but in most instances at lower concentration for the same kinds of woods. The liquor may be continuously supplied to bath D by pipe 52, or in any other suitable manner. Liquid level controls of suitable and well known character may be employed to regulate the level of bath D to insure that the chips are fully submerged, without allowing the liquor to overflow either dam 18 or 19.

A thermostatically controlled heating jacket 53 on the bottom of chamber 11 will serve to maintain the bath D at a temperature above 190 F., preferably between 200 and about 212. Any appropriate heating medium including steam may be supplied in a regulated manner to jacket 53. If desired, some of the hot solvent vapors derived from a still, at later described, may be used to blanket and heat the bath, supplementing the function of jacket 53.

In conventional practice a typical sulfate cooking liquor contains caustic soda and sodium sulfide, and may contain small quantities of other sodium compounds, particularly the carbonate, sulfate and thiosulfate. The latter three are not considered primary factors in sulfate cooking as the cooking agents are essentially the caustic soda and sodium sulfide.

of well known character or other suitable equipment capable of separating the remaining solvent for re-use and the resins as by-products. Such equipment is indicated merely diagrammatically as 71.

The solvent vapor drawn from the still through pipe 69 may be used selectively in various proportions and regulated by valves not shown, by conducting some of it through pipe 72 for introduction into the inlet end of immersion chamber 11 where it will supply heat to bath A and form part of the solvent phase of the azeotrope vapor formed therein. Another portion of this solvent vapor can be deliveredlthrough pipe 73 into the space above bath C to furnish heat to that bath and form part of the azeotrope vapor formed therein.

Baffles 74 and 75, supported in any suitable manner, not shown, will retard the flow of solvent vapor toward bath D but permit azeotrope vapors arising from bath D to be drawn forwardly between the bafiles to the vapor I outlet pipe 54. It should be stated that the solvent vapor The chemical ingredients of the cooking liquor are .5

customarily incorporated in an aqueous solution, hence, in the practice of my invention the water in this solution is available to form an azeotrope vapor at 190 F. with the solvent which has saturated the wood chips prior to their entrance into bath D and is now diffused outwardly to form the azeotrope. The temperature of bath D is suflicient to induce this action. In this manner the solvent is continuously extracted from the Wood chips as the azeotrope vapor is formed and this vapor rises in the chamber 11 and is exhausted through the pipe 54 and pipe 36 into the condenser 37 and disposed of, as heretofore explained.

This action of extracting the solvent from the chips continues in the bath D until the solvent is completely removed and is replaced within the chips by the cooking liquor itself, which then is in position to perform its function of digesting the lignins when the chips are later subjected to pressure and a higher temperature.

The chips leave the bath D upon being dragged by the paddles over the dam 19, into and through a liquid seal for discharge to a storage tank, later described. This liquid seal is established by providing a liquid seal box generally indicated as 55 in which a bath of the cooking liquor will be maintained at a level somewhat below the top of dam 19. An imperforate rotatable drum 56 guides belt 12 down into the bath in the box 55 carrying the chips therewith, and as the belt rises to pass over the roller 57 the chips are dropped by the paddles and released into this liquid seal bath to float upwardly and out through the passage'58 leading to a storage tank 59. An end wall 63 extending into and below the liquid level in the box is spaced from the belt to allow chips to fall back from the paddles into this bath and furnishes an end seal for the chamber 11,

A continuously operating pump 61 draws liquid sol- I vent from bath A through a screen 62, .which prevents escape of wood chips and particles thereof, thence over the liquid level regulating baffle 63, delivering the solvent through pipe 54 to a heat insulated still 65, which is provided with a steam coil 66 and a suitable steam condensate trap 67.

The steam coil heats the solvent to above its boiling point, progressively concentrating in stages the dissolved resins and terpenes. This still should be well insulated or steam jacketed to prevent reflux in the still.

The solvent, containing resins dissolved therein, flows progressively over the cascade type dams such as 68. The solvent vapor is continuously drawn oif through pipe 69, while the highly concentrated solution of solvent and resins in the last and shallowest compartmentin the still can be drawn off through overflow pipe 70 and de; livered to a suitable reboiler and fractionating column is heavier than air, or water vapor, or the azeotrope vapor and will tend to blanket any bath it may be able to reach.

i 'To protect bath B from being contacted and heated to any substantial extent by higher temperatured solvent or azeotrope vapors I prefer to interpose imperforate baflle plates 78 and 79, supported in any suitable manner, not shown, between the upper and lower traverses of the endless belt, and above these baflles are additional bafiles 80 and 81 supporting hinged pendulus flaps 82 and 83, which flaps may be deflected momentarily by the paddles 13.

It may be observed that while a large amount of solvent is utilized in various important ways in the process its manipulation does not require that any of it be wasted. In fact, if the invention be practiced as described in a proper manner there should not be even any unintentional loss of solvent.

Reference will nowbe made to the removal from the immersing chamber of the liquor saturated chips and their transfer to the digester.

The storage tank and liquid seal box may be supplied with cooking liquor by means of a pipe 52 under any suitable liquid level control means, not shown, to maintain a level in both below the top of dam 19, if desired, and high enough to immerse the lower side of drum 56. The chips, saturated with cooking liquor, float into the storage tank 59, which may have any desired capacity, and may be removed therefrom at any desired and regulated rate for transfer into one or more digesters. Apparatus for effecting this transfer in a preferred manner is shown in FIG. 2 and will now be described. Other means may be employed if desired.

An endless belt 85 equipped with rigid transverse paddles 86 is carried by rotatable drums and continuously driven in any suitable manner, not shown, for engaging the floating layer of chips and dragging them up the inclined wall 87 and discharging them into hopper 88. This hopper is connected by its venturi throat 89 to a digester housed within a cylindrical steel shell 90 hav- .ing closed end walls 91 and 92.

The digester tube is preferably made of flanged sewer tile sections 93, cemented together by acid-proof cement. A spiral ribbon flight conveyor screw 94 is supported on and operatively connected to a drive shaft 95, both the screw and the shaft terminating at one end just short of an overflow baffle 96 adjacent a chip receiving basin 97. The other end of the shaft is connected by a universal joint or coupling 98, of any desired construction, to the shaft .99 of motor 100. Anappropriate seal or stuffing box 101 will be provided where the motor shaft extends through the end wall 92 of the steel casing.

Steam at -150 lbs. pressure or at any pressure appropriate for the quality of the fibre being produced may be supplied to the interior of the digester tube by pipe 102 or in any other desired manner.

The axis of the digester is inclined with the inlet end 9 lower to facilitate drainage of condensate and liquor toward the inlet end.

To overcome the high pressure within the digester the liquor saturated chips are introduced thereinto by injector action, although other means to accomplish this purpose could be employed. A screen 103, positioned as shown, permits drainage of liquor to pump 104, which circulates liquor at high velocity through pipe 105, through the constricted nozzle 106 terminating at the entrance of the venturi passage. High pressure steam supplied by pipe 107 aids the venturi action and assists the liquor stream to entrain chips from hopper 88 and inject them into the digester against the internal steam pressure.

The chips and liquor jet fall into the receiving basin 97, overflow bafile 96, and the chips are engaged by the ribbon flight conveyor screw and advanced, while the liquor returns through screen 103 to pump 10 4.

Should the liquor recirculating through pipe 105 become so highly heated as to impair the effectiveness of circulating pump and the steam jet 167, some of this liquor can be diverted into tank 59 and an equivalent amount of cooler liquor from that tank introduced into hopper 36 by the addition of suitable pieces in an obvious manner.

To avoid entraining atmospheric air with the chips at the venturi a pipe 108 may be employed to bubble exhaust or low pressure steam up through the bath in storage tank 59, and, since this tank will be well hooded by the side walls and a cover in a conventional manner, an end wall M9 and bafile 11% air can readily be dis placed from the tank and hopper. Such exhaust steam as may be entrained with the chips will perform useful heating.

Air and other non-condensible gases may nevertheless enter or accumulate in the digester and these should be continuously ejected by one or more conventional air ejectors such as 111.

The chips are removed from the digester after they fall into hopper 112 with the aid of a stream of water from any suitable source supplied through pipe 113, flushing the chips through barrel valve 114, aided by the steam pressure, and thence into pipe 115. The speed of rotation of barrel valve 114 may be regulated by a brake, governor, or in any other suitable manner, or may be positively driven. The chips may now be delivered to any suitable equipment, not forming a part of this invention, for separating, washing and screening the cooked pulp, and for recovering the liquor therefrom, or for other procedures well known or appropriate and useful at this stage.

Referring to the digesting action itself, the chips are moved slowly through the digester and subjected for any necessary period of time to digestion in the hot steam atmosphere under action primarily of the liquor with which they are saturated. Should it be desired to supplement or fortify the liquor in the digester at any point therein, this may easily be effected in an obvious manner, such as by sprays at intervals. After the removal of the chips it will be found that the lignin is substantially completely dissolved and the chips become a spongy mass from which the cellulose fibers are readily removed during subsequent operations. Noting that the prior solvent extraction should reduce the resins and terpenes to less than .5 of the bone dry weight of the chips, the digesting liquor will contain practically no dark resinous soaps, eliminating this undesirable feature customary heretofore in the sulfate or other alkaline processes. Accordingly, the cellulose fibers, not being subjected to action of such soaps, will emerge from this portion of the process much lighter in color than they have emerged from prior alkaline processes. Consequently, less bleaching will be required for producing separated cellulose fibers of desirable light color.

In the prior chemical processes the chips, when first contacting the heated cooking liquors, contained their usual or normal content of moisture, resins and terpenes. Because of the liquor dilution ensuing from the chip moisture entering the liquors it has been customary to start digestion with extra strong liquor in expectation of this dilution. Accordingly, the fibers in the outer portions of the chips from the beginning were subjected to an undesirably severe digesting action and were invariably weakened and destroyed to a greater extent than the fibers which were shielded inside the chips.

In accordance with the invention, the chips are quite uniformly saturated with liquor before the digesting action begins at high temperature and pressure in the digester. Consequently, all fibers are subjected more nearly to a uniform digesting action for the same period of time, and none of them need be subjected to severely destructive action. The net result is fibers of greater strength and uniformity than have heretofore been produced commercially.

To facilitate lifting the screen belt 12 and perforated paddles over the dams 16 to 19 while draining the bath liquids from the chips, I may employ the parts shown in FIGURES 3 and 4. For the sake of clarity these parts are shown only as associated with darn 18, but may be duplicated at each dam.

The tank or chamber 11, for this purpose, will have a bottom wall 120, and inside liquid retaining walls 121 and 122. Outside walls 123 and 124 cooperate with the cover 125 to form a closed chamber. The upper edges of walls 121 and 122 curve upwardly at each dam to retain the paddles and chips. Adjacent each dam idler rollers 126 and 127 will be engaged by the angular brackets such as 128 which are secured to opposite ends of each paddle, and extend with clearance at all times over the walls 121 and 122. These brackets, upon engaging and riding over the rollers, lift the belt, yet keep the paddles in contact with each dam. Other appropriate means for lifting the belt over the dam may be employed.

The details of a positive driving mechanism for the belt are not shown, as any one of a number of suitable mechanisms well known in the conveyor art may be employed in this apparatus.

FIG. 5 shows diagrammatically a somewhat modified flow system for the solvent baths and the azeotrope vapors.

The structure of the immersion chamber shown in FIG. 5 is substantially identical with the chamber 11 shown in FIG. 1. Such differences as there are will be mentioned. This chamber, of which only a part is shown, is designated as 11' and the successive bath solvent separating dams are identified as 16', 17 and 18'. The azeotrope vapor will be withdrawn from the chamber 11 through pipe 36'. A water cooled condenser 37' may serve to condense the azeotrope vapor and the condensate will flow into a gravity separator 39. A carbon adsorber (not shown in FIG. 5) will be connected with the separator '39 in the manner shown in 'FIG. 1 .and will serve the same purpose. As a (litterence from FIG. 1, the liquid solvent emerging from the separator through the pipe 41 selectively may be passed through the heater 42 and returned to the chamber through pipe 43', or the solvent may be diverted through pipe 13d to flow directly into and through bath B, this being advantageous since this liquid solvent will be at a temperature below the azeotrope boiling point and may readily have been cooled in the condenser 37' to Well below l185 P. All or any selected proportion of the condensed solvent may be diverted through pipe 130.

This liquid solvent in bath B performs the previously described function of chilling the chips entering bath B to condense the residual vapor therein and allow the solvent to penetrate therein.

It may be assumed that the chips are advanced in the apparatus of FIG. 5 by a belt (not shown) from bath A to B to C as explained in connection with FIG. 1. In this modification bath B' will overflow through pipe 131 through a temperature regulated heater 132 supplied with any suitable heating medium, for example, steam. The heater will heat the solvent to a temperature well above 190 F. and approaching 250 F. and discharge it through pipe 133 into bath C where the solvent will be further heated by contact with the hot solvent vapors flowing thereover in the manner disclosed in FIG. 1.

In contrast to the flow system of FIG. 1 it will be noted that bath C overflows directly to bath A through pipe 134 without need for intermediate heating, as both of these baths should be at a high temperature, which will be alfected by the evaporation rate of the azeotrope. As the water content of the chips decreases, the bath temperature will more nearly approach 250 F. or whatever other temperatures have been selected as the desired high temperatures for either of baths A and C.

Removal of the solvent containing dissolved resins and terpenes from bath A' and its subsequent distillation may be conducted as shown and described in connection with FIG. 1.

All of the distilled solvent vapor returned to the apparatus of FIG. 5 may well be returned directly over the digester liquor bath, and vapor blocking baffles such as 78 to 83 need not be provided.

Whereas, in FIGS. 1 and 5 the pipes for maintaining the liquid levels in baths A, B, C, A, B and C and conducting flow between them, as shown for clarity, would interfere with the paddles 13-, in practice these pipes may open through side walls 121 and 122.

As a further difference from the disclosure of FIG. 1, by opening valves 135 and 136 and closing valve 137, FIG. 5, the azeotrope vapor withdrawn through pipe 36 may, if desired, be diverted through pipe 138 into the upper end of a chip conveyor tube 139 of any suitable length and diameter to be condensed directly upon the relatively cool fresh chips before they are introduced into the immersion chamber. Preferably, the conveyor tube has a sealed connection with the hopper 32 to prevent escape of solvent to the atmosphere. The conveyor tube 139 may have, if desired, a suitable motor driven conveyor screw 140 for propelling a continuous supply of fresh chips at a regulated rate into the inlet hopper 32. The fresh chips fed into the conveyor through hopper 141 and barrel valve 142 normally will be at a temperature equal to or near that of the surrounding atmosphere, which temperature, of course, may vary widely. The latent heat of the azeotrope vapor will readily be absorbed in any event by the fresh moist chips, and the resultant azeotrope condensate of water and solvent can be drained through the screen 143, through pipe 144 into a pump 145, and delivered by a pipe 146 for passage through condenser 37 or directly into the gravity separator 39'. If the condensate needs cooling it may be passed through the condenser 37. It should be borne in mind, however, that if the chips have the capacity to absorb much more moisture, it might be disadvantageous to condense the azeotrope vapor on them.

Uncondensed vapors and non-condensible gas, if any, will be withdrawn through pipe 147 into the condenser 37 by the action of a suction fan such as 47 (FIG. 1) which will be connected to the separator 39 in the same manner and for the same purpose as in FIG. 1.

The extraction of moisture from the Wood during immersion in the first heated solvent bath is the presently preferred method of moisture removal. However, part or all of the moisture may be extracted by one or more other methods in combination or not with the above preferred method. For example, some of the moisture normally present in the chips may be extracted, before immersing the chips in the bath, by hot stack gases resulting from the burning of the black digester liquor concentrate and from boilers, or other gases preferably low in oxygen may be used. Also some moisture could be extracted in a steam or solvent vapor atmosphere. The use of stack gases would be more practical on woods containing little or no terpenes in order to avoid unnecessary terpene loss.

In FIG. 6 a modified form of apparatus is disclosed. This form differs from that shown in FIG. 1 primarily in respect to the manner in which, following a partial dehydration of the chips, the vapors in the chips are condensed in order to facilitate absorption by the chips of hot solvent. Whereas, as illustrated in FIG. 1, this condensation is accomplished by temporarily immersing the partially dehydrated chips in a cooler bath, that is, cooled below the azeotrope boiling point, in FIG. 6 this objective is accomplished by temporarily immersing the chips in a deep well, deep enough so that the hydrostatic pressure exerted on the chips in the lower portion of the well is sufiicient to condense any azeotrope vapor contained therein, whereupon the solvent will then be drawn into the chips.

Accordingly, an elongated tank 161, of rectangular vertical cross section is provided and three paddle equipped belts are provided therein, such as 162 in the end of the tank where the chips enter, followed by a belt 163 which immerses the chips in the deep well 164, and the third belt 165 which propels the chips through the solvent bath, through the digester liquor bath and through the liquid outlet seal. The belt 162 is preferably a perforated belt such as could be made of screen wire and is equipped with paddles 166 fixed on the belt perpendicular thereto. Drums 167 and 168, supported in any suitable manner, guide this belt so that its lower traverse is submerged in the solvent bath. Means, not shown, are provided for continuously rotating belt 162.

The belt 163 is driven by power means, not shown, and is guided on three or more drums, such as 169, 170 and 171, which are supported in any suitable manner, and which cause the belt to have a substantial horizontal transverse in the botom of the deep well. Paddles 172 are likewise secured on the belt in a position whereby the propelling face, such as 173, is maintained perpendicular to the belt while the rear curved face 1'74 is so positioned that during the upward movement of the belt paddles from the well the curved rear face will tend to free the chips from any tendency to collect behind the paddles.

The third belt 165 will be provided with paddles 175 similar to those on belt 162 and will be assisted in riding over the bath separating dams 176 and 177 by mechanism of the character shown in FIGS. 3 and 4. Belt 165, like belt 162, travels with its lower traverse immersed in the liquid baths so that chips held down by the belt Will be held submerged, excepting when riding over the dams.

The depth of the well 164 is subject to some variations, being correlated with such factors as the specific gravity of the solvent employed, and the temperature differential between the azeotrope boiling point and the temperature of the solvent bath in the deep well. When using perchlorethylene, for example, whose azeotrope boiling point with water at atmospheric pressure is 190 F., the hydrostatic pressure at the bottom of the well should be great enough so that the azeotrope boiling point at such increased pressure is less than the solvent bath temperature. The solvent bath may be maintained at any temperature substantially above such increased azeotrope boiling point to afford an effective temperature differs ntial for heat transfer, for example, between 200 F. and 250 F.

A pipe 178 may be employed to supply digester liquor continuously to a liquor bath located between dams 176 and 177, which bath is separately heated by the steam jacket 179 to a range of about 200 F. to 210 F. The belt 165 may be supported on a plurality of drums such as 180 and will be caused to pass through the liquid 13 seal 181 by means of a drum 182 for the same purpose described in connection with FIG. '1.

When utilizing the device shown in FIG. 6 the chips will be introduced through the hopper 183 and the barrel valve 184 and will emerge eventually through an outlet channel 185 into a tank such as 59 (FIG. 2) for the purpose of being injected into the digester chamber, such as is shown in FIG. 2.

Preferably, the azeotrope vapors withdrawn from the device shown in FIG. 6 will emerge through a pipe 186 into a condenser 187 with which is associated a gravity separator 188 and a carbon adsorber 189, the functions of which, together with other associated apparatus, will be understood upon comparison with FIG. 1 and the description thereof.

The solvent, containing resins dissolved therein, will be continuously withdrawn from the chamber 161 over a bafiie 190 by meansof a pump 191 and delivered by pipe 192 to a solvent still 193 whose construction and operation will be understood upon comparison with FIG. 1 and the description thereof. The distilled solvent vapor will be delivered from the still by pipe 194 into the immersion chamber preferably in the manner shown, while the liquid solvent removed from the separator 188 will be heated by the heater 195 and returned to the process by pipe 196.

Baffles such as 197 and 198 will be positioned as shown and supported in any suitable manner not shown to prevent the hot solvent vapors from flowing back toward the digester liquor bath. However, azeotrope vapors formed in and arising from the liquor bath will be drawn forwardly between the bafiies by the suction action exerted through the azeotrope vapor outlet pipe 186.

A distributing baffle 199 may be mounted beneath the barrel valve 194- to serve the same purpose as the baffie 34 shown in FIG. 1 and a spring loaded baffle 2&0 will correspond in location and function to the bafiie 34' of FIG. 1. An additional bafile 201 located as shown and suitably mounted will enable the belt 165 to engage and hold in a submerged path of travel the chips floating upwardly from the deep well.

In the operation of the device shown in FIG. 6, the chips upon being emmersed in the hot solvent underlying belt 162 will have moisture expelled from them to any desired extent, this moisture escaping from the tank as an azeotrope vapor, as will now be readily understood. The chips advanced by belt 162 will be transferred to belt 163 and carried downwardly into and through the deep well. The increased hydrostatic pressure in the deep well is effective to condense the azeotrope vapors in the chips and enable the solvent to penetrate the void caused by such condensation. The length of the time the chips are subjected to this higher hydrostatic pressure may be varied by such means as varying speed of travel of the deep well belt, or by varying the length of the horizontal traverse of the belt in such deep well. As soon as the chips are carried around the drum 171 they will be released from the control of the belt paddles 172 and will accordingly flow upwardly through solvent bath to a position underneath'the submerged traverse of the belt 165. After suitable penetration of the chips by the solvent the solvent extraction of the resins will proceed in the manner heretofore described. The progress of the chips thereafter through the remainder of that solvent bath and through the digester liquor bath will be understood from the previous description.

In FIGURES 7 and 8 is illustrated a modification of the invention in which the chips are moved successively and counter-currently through three solvent baths, lifted from the third and then conveyed and tumbled through a zone containing superheated solvent vapor which operates to vaporize from the chips all liquid solvent then carried by them. Thereafter, while still superheated, the chips are immersed in a digester liquor bath in which they become purged of solvent vapor, the liquor dis- '14 placing the solvent within the chips, thereby saturating the chips with liquor.

The construction and use of the apparatus of FIG- URES 7 and 8 may be described rather briefly as it employs much of the apparatus previously described, especially in connection with FIGURE 1.

In FIGURE 7 there is shown diagrammatically an elongated chamber 210, which may be of generally rec tangular cross section, sealed from the atmosphere generally, and adapted to deliver liquor saturated chips into the digester apparatus of FIG. 2, as does that of FIG. 1.

The chips to be treated are introduced into hopper 211, pass through barrel valve 212, and drop upon the top traverse of a perforated endless belt 213. The paddles 214 of this belt, preferably, as are those of FIG. 1, are perforated, or made of screen Wire, as may be the belt. The belt and its paddles immerse the newly introduced chips in the first hot solvent bath, which should have a temperature preferably in the higher end of a range between the azeotrope boiling point and the solvent boiling point. This immersion treatment is sufiicient in duration to evaporate most of the moisture from the chips and to dissolve much of the gums, resins, etc., as heretofore described.

The belt then drags the chips over dam 215, draining them of solvent, after which they are immersed by the belt in the second solvent bath, which has a temperature well below the azeotrope boiling point. This bath may conveniently be supplied continuously with cool solvent from the gravity separator 216, similarly to the correspondingly feature of FIGURE 5. Here the vapors in the chips are quickly condensed, enabling the liquid solvent to penetrate deeply into the chips.

The belt then drags the cooled chips up over dam 217 and immerses them in another hot solvent bath, wherein they remain long enough to evaporate additional moisture from the chips and to dissolve substantially the remainder of the resins, gum, etc., from the chips or to the extent desired. This bath, Whose temperature should be about the same as that of the first hot solvent bath, is continuously supplied with fresh heated solvent passing through heat exchanger 218, and is further heated by hot solvent still vapors from pipe line 219. The overflow of solvent from the second bath is also heated by heat exchanger 22a en route, as shown, to the third bath by means of pipes 221 and 222. The overflow of hot solvent from the third bath is conducted by pipe 223 to supply the first bath, which also is partially heated by hot solvent still vapors delivered by pipe line 224. Any of these baths, as well as the subsequently described liquor bath may be jacketed for heating or insulating purposes, as in the other forms of the invention.

As will be readily understood from the preceding portions of this specification, a stream of resin-containing solvent, passing over the liquid level control baffle 225, is delivered by pump 226 and line 227 to the cascade type solvent still 228 for distilling and recovering solvent from the resins. The resins may be recovered in a conventional fractionating column or other suitable equipment indicated by numeral 229. Lines 230, 231 and 232 collect azeotrope vapors, solvent vapors and non-condensibles from the several zones for introduction by pipe 233 into condenser 234, whose functioning in cooperation with Water separator 216, carbon adsorber 235, blower 236, and associated equipment will be understood by reference to previous description herein of similar equipment.

Reverting to the progressive treatment of the chips, the belt 213 elevates and drains the chips while propelling them up over dam 237 and discharging them onto the first of a series of endless conveyor belt such as 238, 239,

and 240.

Whereas divided baflies 241 and 242 serve to protect the cool solvent bath from the hotter vapors existing above the other two solvent baths, I provide a divided bafile 243 and a one piece bafiie 244 to confine superheated solvent vapors in the zone surrounding the conveyors 238-240. As the chips are advanced and tumbled by these conveyors they lose some liquid solvent by drainage to the floor of the chamber and this is evacuated by pipe 245 to the pump 226. Pipe 246 and blower 247 evacuate solvent vapors from the space beneath the conveyors and cause them to pass through a heat exchanger 248 which superheats them preferably (when perchlorethylene is the solvent) to a range near 300 F., pipe 249 returning these vapors to the chamber. Thus, the superheat in this vapor is employed to vaporize from the surfaces and interiors of the chips substantially all of the solvent then present. As a small positive pressure may be created in this zone by the superheat, some of the excess vapor may be allowed to blow through the divided baffie 243 and some may be relieved, if desired, by a conventional back-pressure valve 250 into pipe line 233 for delivery to the condenser 234 under the negative pressure induced in part by blower 236.

When conveyor 240 dumps the chips upon conveyor 251 the latter carries them down into a digester liquor bath maintained at a temperature between the azeotrope boiling point and the water boiling point. For example, when perchlorethylene is the solvent being used, the range is 190 F. to 212 F. and by proper jacketing and temperature control the bath may be maintained preferably between 200 and 210 F. Immersion of the hot chips in the relatively cooler aqueous liquor bath purges solvent therefrom, the solvent forming an azeotrope vapor with water. As the hot solvent vapor is removed from the chips, the liquor penetrates them and saturates them. Not only is solvent purged from the surfaces of the chips, but absorbed solvent therein also forms an azeotrope with the water in the liquor, escapes, and is evacuated through pipe 232.

Conveyor belt 251 supported on rollers 252, 253, 254 and 255 is guided under the baffie 244 by idler discs such as 256 and 257 contacting the protruding ends of reinforcing bars 258 on the belt. Belt 251 lifts the chips out of the first liquor bath, allowing them to drain as they emerge, and drops them onto another belt 259, which carries them into another digester liquor bath and underneath vapor bafile 260. When this belt releases the chips they emerge into channel 261 which may, if desired, be employed to deliver the liquor-saturated chips into the tank 59 and associated digester apparatus shown. in FIG. 2.

Digester liquor may be supplied to the liquor baths, to

maintain the levels indicated by the dotted lines, by pipes 262 and 263 together with conventional liquid level control devices, not shown.

Solvent may be supplied initially to the solvent bath system through pipe 264 and valve 265, but it should be understood that replenishment of solvent will be little required as this process need not suffer any substantial solvent loss.

In a conventional manner, valves such as 266 and 267, or other suitable means, may be employed to proportion the liquid solvent flow from separator 216 while valves 268 and 269, or other suitable means, may be employed to proportion the solvent vapor flow derived from still 228.

The flow diagram of FIGURE 7 is condensed for convenience and clarity of illustration, hence no inference as to relative sizes of apparatus should be drawn therefrom.

The solvent bath or medium in which the chips are placed prior to absorption of the cooking liquor may be composed of some halogenated hydrocarbon solvents mentioned herein other than perchlorethylene, and the wood treated need not be in the form of chips which are now commonly used in pulp processing.

The physical conditionof the wood chips lends itself admirably to various features of this invention. the wood is chipped the fibers and the minute channels When which largely parallel the fibers are cut transversely of the chips are usually not over one inch in length, meas ured parallel to the fibers and channels, both the extraction and penetration heretofore discussed can readily be accomplished.

While saturation of the wood chips by the cooking or digester liquor will vary with difierent woods and may be varied by intentional manipulation of the process of the invention, it has been observed that some of the pulp woods readily absorb from to 200% of their dry weight, which in many instances is sufficient liquor for adequate digestion of the chips.

Though specific reference is made herein to the processing of wood chips to produce improved fibers for paper manufacturing and other uses, the principles of the invention may be used with advantage in the processing of other vegetable substances containing cellulose fibers.

With regard to the solvents which may be used in this invention, a number of them can be employed. To be useful efficiently in this invention the solvent should be immiscible with water, it should be a good solvent for the natural wood resins, it should be nonflammable as a matter of safety, it should form an azeotrope with water, in liquid form it should have a specific gravity diiierent from that of water, there should be a substantial temperature differential between the azeotrope boiling point and the solvent boiling point so that the heated solvent bath may effectively or efficiently supply the heat for vaporizing the azeotrope without danger of the solvent boiling off otherwise than in the azeotrope. The solvent also should be a good heat transfer medium. Low toxicity when inhaled is desirable as a safety measure. The ratio of solvent to water in the azeotrope vapor should be low enough that the total heat required to evaporate a pound of water in the azeotrope vapor will not be so unreasonably high as to make the heating costs commercially prohibitive.

Among the solvents which meet the above standards are a group of chlorinated hydrocarbons, sometimes termed chlorohdyrocarbons, containing the following members:

Trichlorethylene Perchlorethylene Ethylene dichloride omol Tetrachlorethane Pentachlorethane Of this group perchlorethylene is preferred, because of its various physical properties, performance characteristics, commercial availability and price.

The above listed chlorinated hydrocarbonsare halogenated hydrocarbons in which two or more hydrogen atoms have been replaced by chlorineatoms.

Other halogenated hydrocarbons, not all of which are presently commercially available but which are believed to be resin solvents and may be otherwise suitable, which may be usedin this invention if they are suitable, are certain fluorinated hydrocarbons sometimes called fluorohydrocarbons in which two or more hydrogen atoms have t7 been replaced by fluorine atoms. Among these fluorohydrocarbons are the following, identified by their formulas and their boiling points:

C. CF s2 flFlh n-c mc r 9s n'cqHzclzF 'o 1 Ii-CqHzCgFg Il-CqHgClgFg 1 3 n' rHa iz 125 (3 01 1- 91-92 3 141 111-5 c nons, 127-129 sClSFa 152 0 2101 5 109410 C 'HC1' F ll0l l2 8 1 it should be recognized that in the use of any of the apparatus or forms of the invention above disclosed, the wood chips being treated may vary greatly in their content of resins, gums and terpenes. Nevertheless, even though some of the chips in single or continuous charge may have a high resin content andothers little or no resin, theprocesses described herein will uniformly con dition the chips for liquor absorption preparatory to diges'tion; Inaddition to removing the air and moisture rrom the chips; anything else which they contain, which may be'removed by evaporation or by solvent extraction, will also be removed, the chips will become saturated with. solvent and subsequently by substitution will become saturated with" digester liquor before digestion begins. Consequently, the invention in all of its forms may be used'with great success to prepare for digestion and to digest wood chips commonly regarded as having great or negligible resin content.

While the continuous processing of wood chips and their digestion has manifest advantages, there may be some plants having a large investment in digesters which the owners feel should be used, if possible, or as far as possible, and this invention" lends itself readily to such adaptation;

For example, the process steps performed on continuously supplied chips in the apparatus shownin FIGURES 1 and 2 may be performed in modified form on batches or charges of chips in a'c'onventional digester, into which the wood chips are customarily introduced at the top, having one or more pipes provided at the bottom for Withdrawing the contents. To accomplish this, the digester would be charged with chips and one of the aforementioned solvents, for example, perchlorethylene', would be introduced into the digester. If the chips have a resin content which shouldbe removed, all the chips should be completely immersed, but if the resin content is negligible, the chips need not be completely immersed, though nearly so preferably.

It is conventional for most digesters to have one or more valved pipes entering the top of the digester for various purposes, such as for re-introducing recirculating digester liquor and for venting gases and steam.

One such top pipe having a valve and having a screen between the valve and the top of the digester/for preventing outward escape of chips would, for the batch practice of this invention, be connected to a pipe such as the pipe 36 shown in FIGURE 1, leading to the solvent recovery system consisting of the condenser 37, the gravity separator generally indicated as 39 and the carbon absorber 46, with which are associated the fan 47 and the related pipes and valves.

7 When the digester has been filled with chips-and solvent, it is then heated by conventional coils or jackets, (preferably not by steam introduction) to a temperature range above the azeotrope boiling point of the particular solvent, in this case, F. for perchlorethylene, but below 250 E, which is the boiling point of this solvent. This operation will cause moisture in the chips to evaporate and as it emerges from the chips it will form an azeotrope vapor with the solvent and be withdrawn through pipe 36 into the solvent recovery system wherein the water and air will be separated from the solvent and the latter condensed tor reuse, as will be understood [from the foregoing description.

As moisture leaves the chips, solvent will penetrate them and, if there be resin present, the solvent extraction of the resin will begin. When the flow of water from the gravity separator diminishes to a very small amount, as may be observed readily, this indicates that moisture has been substantially completely extracted from the chips. To further indicate the end of the chip dehydration, a thermometer may be inserted into pipe 36 to indicate the vapor temperature therein, it will show a sudden temperature increase when the azeotrope vapor diminishes to a small flow and is supplanted by solvent vapor.

When the chips have been sufiicie'ntly dehydrated, the above mentioned valve provided in the digester outlet pipe connected to the pipe 36, may be closed to seal the digester and the pressure therein may be raised, which will occur naturally if the heating is continued and no vapor escapes. If desired, increase in pressure may be aided by pumping in either solvent or solvent vapor. The increase in pressure will cause further penetration of solvent into the chips to facilitate resin extraction, if there be resin. Cooling of the solvent might be used to facilitate its penetration but might be ineth'cient trom' a cost standpoint. As long as there be more resin or rather a higher concentration of resin, in the chips than there is in the surrounding liquid solvent, the difference in osmotic pressure will cause the resin to move outwardly from the chips into the solvent bath, and this will occur whether or not the digester is under pressure. When, as may be learned from experience with any particular'type of wood, adequate resin extraction has been accomplished, the pressure on the digester, if any, may be released by opening the valve to the solvent recovery system, or otherwise, causing some of the solvent to flow out of the chips, carrying with it resin, leaving the chips, however, still saturated with solvent, which is desired. The cycles of pressure release and increase may be repeated if desired.

The vapor outlet valve leading to the recovery system should then be closed and the digester then may be drained through a conventional bottom pipe and this solvent conducted to storage for reuse if it is little contaminated by resin. If its resin content is high, such solvent may be conducted to a device such as the resin still' 65, in which the solvent may then be distilled from the resin, and both separately recovered, asindicated heretofore in connection with FIG. 1. While the digester is being drained of solvent, it is preferred to introduce into the top of the digester solvent vapor of the same solvent, which can easily be produced, as a substitute for the withdrawn liquid solvent, to avoid subjecting the chips to reduced pressure, and to prevent the entrance of air into the digester at this time. At this time, the wood chips are free of resin and moisture arid are saturated with solvent.

The next step after the draining of the solvent is to introduce digester liquor through the bottom of the digester while simultaneously expelling the solvent vapor at the top. in an obvious manner, this expelled solvent vapor may be condensed and recoveredin the solvent recovery system.

The contents of the digester will then be heated to above 190 F., the aforementioned azeotrope vapor boiling point, but preferably below 212 F., in order to diminish the boiling off of large quantities of water, and to avoid unnecessary heating of the chips before they become saturated with digester liquor. Actuahy, the average temperature in the digester will not rise much above 190 F., while solvent is being evaporated, hence it is well below conventional temperatures used for digestion. It should be noted that conventional digester liquors employed in the various pulp making processes, any of which liquors may be used in connection with this invention, are aqueous liquors. In this stage of the operation, the perchlorethylene is evaporated from the chips and forms an azeotrope vapor with some of the water contained in the liquor. This azeotrope vapor will be continuously withdrawn through the re-opened valve leading to pipe 36 and the solvent recovered from it in the condenser-liquid separator-carbon adsorber system, as will be well understood.

If it be desired that the concentration of liquor in the digester be kept constant, the water recovered in the separator 39 at this time may be returned to the digester.

As the solvent evaporates from the chips, it is replaced therein by digester liquor in a uniform manner, thus completely saturating the chips before digestion begins. When from observation of the operation of the gravity separator it is noticed that the flow of water and solvent has diminished to a trickle, at that time it will be known that substantially all or all of the solvent has been recovered from the digester and the chips are now entirely saturated with liquor. A sharp temperature rise in pipe 36 may also be noted for indication of this condition.

The solvent recovery system will then be shut oif, the digester sealed, and its temperature and pressure raised in a conventional manner to whatever digesting temperature the operator may desire. The increased pressure facilitates immediately more complete saturation of the chips by the liquor prior to the beginning of the digesting action. The solvents here employed, among which perchloroethylene is preferred, have a peculiar advantage which I have discovered of conditioning the chips so that they will soak up digesting liquor more readily and more thoroughly than has heretofore been experienced in this art, to the best of my knowledge and belief.

Because of the superior conditioning and treatment of the chips which this invention provides, it is expected that digesting time may be shortened, lower digesting temperatures may be employed than have heretofore been possible, the cellulose fibers produced will be superior in quality, as explained heretofore, and the total yield of good cellulose fiber per ton of chips will be vastly improved.

When the digestion of the batch has been completed, the digester may then be emptied and reused to process another batch, as above described.

Alternatively, if desired, the liquor-saturated chips which may be produced in the apparatus of FIGURE 1, may be introduced into a conventional digester and there digested as a batch operation.

This application is a continuation-in-part of my application Serial No. 308,414, filed September 8, 1952, and now abandoned.

While preferred forms of apparatus have been diagrammatically illustrated herein and the preferred methods explained, it should be understood that the process steps and apparatus are subject to considerable variation without departing from the principles of the invention, as defined in the claims.

Having shown and described my invention, I claim:

1. In a wood pulp treatment apparatus, a series of solvent chambers and a digester liquor chamber, belt conveyor means arranged for continuously moving wood chips successively through the solvent chambers and into the liquor chamber and holding them submerged below the liquid level in each chamber, means cooperating with the conveyor means for draining liquid from the chips as they are advanced by the conveyor means from one chamber to the next, means for supplying heat to the liquid in each chamber, and means for collecting solvent- Water azeotrope vapor arising from said c m ers and 20 condensing it and returning the condensed solvent to the solvent chambers.

2. In a wood pulp treatment apparatus, a series of solvent chambers and a digester liquor chamber, belt conveyor means arranged for continuously moving wood chips successively through the solvent chambers and 1nto the liquor chamber and holding them submerged below the liquid level in each chamber, means cooperating w th the conveyor means for draining liquid from the chips as they are advanced by the conveyor means from one chamber to the next, means for supplying heat to the liquid in each chamber, means for collecting solventwater azeotrope vapor arising from said chambers and condensing it and returning the condensed solvent to the solvent chambers, a digestion chamber having inlet and outlet means and conveyor means for moving chips from the inlet to the outlet, means for supplying steam under pressure to the digestion chamber, and means for introducing chips from the liquor chamber into said digestion chamber through said inlet means.

3. In a wood pulp treatment apparatus, a series of solvent chambers for maintaining at least three separate liquid baths of solvent, a digester liquor chamber, means for maintaining the solvent baths in the first and last of the series of solvent chambers and the liquor bath in the liquor chamber at a temperature above the solvent-water azeotrope boiling point, means for maintaining the temperature of one solvent chamber bath between said first and last solvent chambers at a temperature below said azeotrope boiling point, and means for submerging and continuously moving wood chips successively through said solvent bath chambers into said liquor chamber together with means for draining free liquid from the chips as they advance from one chamber to the next.

4. In a wood pulp treatment apparatus, a series of solvent chambers and a digester liquor chamber, belt conveyor means arranged for continuously moving wood chips successively through the solvent chambers and into the liquor chamber and holding them submerged below the liquid level in each chamber, means cooperating with the conveyor means for draining liquid from the chips as they are advanced by the conveyor means from one chamber to the next, means for supplying heat to the liquid in each chamber, means including a condenser for collecting solvent-Water azeotrope vapor arising from said chambers and condensing it, means separating condensed solvent from the condensed water vapor, means including a carbon adsorber connected for adsorbing in the adsorber such solvent vapors as are uncondensed in said condenser and discharging through the adsorber non-condensible gases emerging from the condenser, and means for returning condensed solvent to the solvent chambers.

5. In a wood pulp treatment apparatus, a closed housing, a series of solvent vats in said housing and a digester liquor vat, belt conveyor means including a perforated belt having paddles thereon arranged for continuously moving wood chips successively through the solvent vats and into the liquor vat and holding them submerged below the liquid level in each vat, means cooperating with the conveyor for draining liquid from the chips as they are advanced by the conveyor means from one vat to the next, means for maintaining elevated temperatures in the liquid in each vat, and means for collecting solventwater azeotrope vapor arising from said vats and for condensing it and returning the condensed solvent for reuse in the solvent vats.

6. In a process for chemical digestion of wood chips, the steps comprising heating the chips submerged in a halogenated hydrocarbon solvent liquid bath having a temperature between its azeotrope boiling point and its solvent boiling point, whereby and until substantially all the moisture is evaporated from the chips in an azeotrope vapor, thereafter maintaining the dehydrated chips free of re-absorption of air and water and saturating them with said solvent while in a bath thereof, separating the chips from the solvent bath and while still dehydrated and solvent-saturated immersing them in a heated digester liquor bath having a temperature between its azeotrope boiling point and its own boiling point and thereby vaporizing the solvent from the chips in an azeotrope vapor, continuing said evaporation and immersion in the liquor until the chips become solvent-free and are saturated by said liquor, and thereafter digesting the chips in said liquor, the solvent employed being selected from the group consisting of trichlorethylene, perchlorethylene, ethylene dichloride, tetrachlorethane and pentachlorethane.

7. In a process for chemical digestion of wood chips, the steps including heating thechips in a halogenated hydrocarbon solvent bath having a temperature above the solvent-water azeotrope boiling point but below the solvent boiling point whereby and until substantially all of the moisture is evaporated as an azeotrope vapor from the chips, condensing residual vapor in the wood while the dehydrated chips are in said bath of said solvent thereby efiecting absorption of the solvent into said dehydrated chips, thereafter separating the dehydrated and solventsaturated chips from the solvent bath and immersing them while still dehydrated and solvent-saturated in a heated aqueous digester liquor bath having a temperature above the solvent-water azeotrope boiling point but below the liquor boiling point at the existing pressure, evaporating the contained solvent from the chips in the last said bath as an azeotrope vapor While thereby effecting substantially complete saturation of the wood by the liquor in substitution for the evaporated solvent, removing the azeotrope vapors as produced in the process and recovering the solvent therefrom, and thereafter digesting the chips by means of the liquor, the solvent employed being selected from the group consisting of trichlorethylene, perchlorethylene, ethylene dichloride, tetrachlorethane and pentachlorethane.

8. In a process for chemical digestion of wood chips, the steps including heating the chips while submerged in a halogenated hydrocarbon solvent bath having a temperature above the solvent-water azeotrope boiling point but below the solvent boiling point whereby substantially all of the moisture is evaporated from the chips as an azeotrope vapor, condensing residual vapor in the wood while the chips are in a bath of said solvent thereby eflecting absorption of the solvent into said chips, dissolving resins from the chips into said solvent bath and separating the chips from the resin-containing solvent, saturating the chips with resin-free solvent, thereafter immersing the solvent-saturated chips in a heated aqueous digester liquor having a temperature above the solvent-water azeotrope boiling point but lower than the liquor boiling point and evaporating contained solvent from the chips as an azeotrope vapor and eliecting substantially complete saturation of the wood by the liquor in substitution for the evaporated solvent, and thereafter digesting the chips by means of the liquor with which they are saturated, the chips after dehydration and said saturation with solvent being maintained free of reabsorption of water prior to immersion in said liquor, the solvent employed being selected from the group consisting of trichlorethylene, perchlorethylene, ethylene dichloride, tetrachlorethane and pentachlorethane.

9. In a process for chemical digestion of wood chips, the steps comprising heating the chips in a halogenated hydrocarbon solvent liquid bath having a temperature between the azeotrope boiling point and the solvent boiling point until substantially all the moisture is evaporated from the chips, and condensing the remaining vapor in the chips while still submerged in said bath thereby saturating the chips simultaneously with the liquid solvent, dissolving the resins from the chips into said solvent bath, separating the chips from the resincontaining solvent, thereafter saturating the chips with resin-free solvent and immersing the solvent saturated and resin-free chips in a heated aqueous digester liquor having a temperature above the solvent-water azeotrope boiling point but lower than the liquor boiling point and evaporating the residual solvent from the chips as an azeotrope vapor with some of the water in said liquor and simultaneously saturating the chips with said liquor, and thereafter dissolving the lignin in the chips by means of the liquor maintained under pressure above its atmospheric boiling point, the chips after dehydration and said saturation with solvent being maintained free of reabsorption of moisture prior to immersion in said liquor, the solvent employed being selected from the group con sisting of trichlorethylene, perchloethylene, ethylene dichloride, tetrachlorethane and pentachlorethane.

10. 'In a process for chemical digestion of wood fragments, the steps of saturating the wood with solvent while submerged in a halogenated hydrocarbon solvent liquid bath after removal of moisture from the wood, immersing the solvent-saturated dehydrated wood in a heated aqueous digester liquor having a temperature between the solvent-water azeotrope boiling point and the water boiling point at the existing pressure until the solvent evaporates from the wood as an azeotrope vapor and the wood becomes saturated with the -liquor by substitution for the evaporated solvent, and thereafter raising the liquor temperature and digesting the wood in said liquor, the solvent employed being selected from the group consisting of trichlorethylene, perchlorethylene, ethylene dichloride, tetrachlorethane and pentachlorethane.

References Cited in the file of this patent UNITED STATES PATENTS 1,144,171 Clope June 22, 1915 1,224,722 Douglas May 1, 1917 1,410,069 Moriondi Mar. 21, 1922 1,560,446 Wallace Nov. 3, 1925 1,975,161 Kipper Oct. 2, 1934 2,200,034 Merrill May 7, 1940 

1. IN A WOOD PULP TREATMENT APPARATUS, A SERIES OF SOLVENT CHAMBERS AND A DIGESTER LIQUOR CHAMBER, BELT CONVEYOR MEANS ARRANGED FOR CONTINUOUSLY MOVING WOOD CHIPS SUCCESSIVELY THROUGH THE SOLVENT CHAMBERS AND INTO THE LIQUOR CHAMBER AND HOLDING THEM SUBMERGED BELOW THE LIQUID LEVEL IN EACH CHAMBER, MEANS COOPERATING WITH THE CONVEYOR MEANS FOR DRAINING LIQUID FROM THE CHIPS AS THEY ARE ADVANCED BY THE CONVEYOR MEANS FROM ONE CHAMBER TO THE NEXT, MEANS FOR SUPPLYING HEAT TO THE LIQUID IN EACH CHAMBER, AND MEANS FOR COLLECTING SOLVENTWATER AZEOTROPE VAPOR ARISING FROM SAID CHAMBERS AND 